AND MODERN PHYSICS. 221 



displacements of tho labile ether would obey the laws 

 of light, and from tho fundamental hypotheses of the 

 theory, a mechanical explanation, reasonably satis- 

 factory in its main features, can bo given of most 

 purely optical phenomena. Tho relations between 

 light and electricity, or light and magnetism, arc not, 

 however, touched by this theory; indeed, they cannot 

 bo touched without making some assumption as to 

 what electric displacement is. 



In recent years various suggestions havo been 

 made as to tho nature of tho change which constitutes 

 electric displacement. One theory, due to Von Helm- 

 holtx, supposes that tho eloctro-kinetic momentum, or 

 vector potential of Maxwell, is actually the momen- 

 tum of the moving ether ; according to another, sug- 

 gested, it would appear originally in a crude form 

 by Challis, and developed within the last few months 

 in very satisfactory detail by Larmor, the velocity 

 of the ether is magnetic force; others have been 

 devised, but we are still waiting for a second Newton 

 to* give us a theory of the ether which shall include 

 the facts of electricity and magnetism, luminous radi- 

 ation, and it may be gravitation.* 



Meanwhile we believe that Maxwell has taken the 

 first steps towards this discovery, and has pointed out 

 the lines along which the future discoverer must direct 

 his search, and hence we claim for him a foremost 

 place among the leaders of this century of science. 



* Fora wry Htigvri'ttivoiirt.-oiint of some po*>il!< i tlu'orim* ...v.^...v w 

 liouM W in.ulo to tho (iii^itK'iitinl .IMITV of Ihrofos^or \\'. M. Huks 

 .-. S'-on A of t!i % Uritinh AMMN-iatton at Ipnwirh in isr. 



